1. Field of the Invention
The present invention relates to image sensors, and more particularly, to a complementary metal-oxide-semiconductor (CMOS) image sensor and a method for fabricating the same in which the photodiode projection depth is controlled to obtain improved color balance.
2. Discussion of the Related Art
Image sensors are semiconductor devices for converting an optical image into an electrical signal and include charge-coupled devices and complementary metal-oxide-semiconductor (CMOS) image sensors.
A typical charge-coupled device includes an array of photodiodes converting light signals into electrical signals, a plurality of vertical charge-coupled devices formed between each vertical photodiode aligned in a matrix-type configuration and vertically transmitting electrical charges generated from each photodiode, a horizontal charge-coupled device for horizontally transmitting the electrical charges transmitted by each of the vertical charge-coupled devices, and a sense amplifier for sensing and outputting the horizontally transmitted electrical charges. Charge-coupled devices have the disadvantages of requiring a complicated driving method, high power consumption, and a complicated fabrication process with a multi-phased photo process. Additionally, in a charge-coupled device integration of complementary circuitry such as a control circuit, a signal processor, and an analog-to-digital converter into a single-chip device is difficult, thereby their use hinders the development of compact-sized (thin) products, e.g., digital still cameras and digital video cameras, using such image sensors.
CMOS image sensors, on the other hand, adopt CMOS technology that uses control circuit and a signal processing circuit as a peripheral circuit and adopts switching technology which detects an image by allowing outputs to be sequentially detected using MOS transistors provided in correspondence with the number of pixels arrayed. Additionally, a CMOS image sensor uses CMOS fabrication technology, i.e., a simple fabrication method using fewer photolithography steps, and results in a device exhibiting low power consumption.
In the aforementioned CMOS image sensors, typically, the photodiode is the active device for generating an optical image based on incident light signals. In such a CMOS image sensor, wherein each photodiode senses incident light and the corresponding CMOS logic circuit converts the sensed light into an electrical signal according to input wavelength, the photodiode's photosensitivity increases as more light is able to reach the photodiode. One way of enhancing a CMOS image sensor's photosensitivity is to improve its “fill factor,” i.e., the degree of surface area covered by the photodiodes with respect to the entire surface area of the image sensor. Accordingly, the fill factor is improved by increasing the area responsive to incident light. The concentration of incident light onto the photodiode is further facilitated when the quantum efficiency at all wavelengths (white light) is “1,” which represents a balanced transmission to the photodiodes across the spectrum to include complimentary components of red light, blue light, and green light received at the photodiodes.
To concentrate the incident light on one or more photodiodes, a device exhibiting excellent light transmittance, such as a convex microlens for refracting incident light, may be provided. The convex microlens is used to redirect any light that may otherwise be incident to the image sensor outside the immediate area of the photodiodes. In a color image sensor, such a microlens having a predetermined curvature (i.e., a convex lens) may be provided over a color filter layer for passing the light of each color (wavelength). As shown in FIG. 1, a CMOS image sensor according to the related art, includes three photodiodes provided for generating electrical signals according to the intensity and wavelength of incident light.
Referring to FIG. 1, a CMOS image sensor according to the related art includes a p-type epitaxial layer 11 grown on a p-type semiconductor substrate 10 defining a device isolation region and an active region. A field oxide layer 12 is formed in the device isolation region of the semiconductor substrate 10 to isolate light-signal input regions of blue (B) light, green (G) light, and red (R) light from one another. First, second, and third n-type regions 13, 14, and 15, to serve as the respective photodiodes of a color image sensor, are formed of equal depths by ion-implantation in the active region of the semiconductor substrate 10. Subsequently, a series of gate electrodes 17 are formed on the active region of the semiconductor substrate 10 by interposing a gate insulating film 16 patterned with the gate electrodes. Dielectric spacers 18 are formed at the lateral sides of each gate electrode 17. A dielectric interlayer 19 is formed over the entire surface of the semiconductor substrate 10 including the gate electrodes 17. A color filter layer 20 comprised of blue, green, and red filters (i.e., a color filter array) is formed on the dielectric interlayer 19 to correspond to the first, second, and third n-type regions 13, 14, and 15. A planarization layer 21 is formed over the entire surface of the semiconductor substrate 10 including the color filter layer 20. A plurality of microlenses 22 is formed on the planarization layer 21 to correspond to the respective color filters of the color filter layer 20.
In the operation of the CMOS image sensor, incident light is concentrated by the microlenses and received by the photodiodes in each of the n-type regions. During this light signal reception, light of each color penetrates the silicon layer by a predetermined depth according to wavelength, with longer wavelengths, e.g., red light, achieving a deeper penetration.
Since the photodiodes of the CMOS image sensor according to the related art, as described above, are respectively formed of the first, second, and third n-type regions 13, 14, and 15, they each have the same projection depth regardless of the intended spectral reception, i.e., red, green, or blue light. However, because the penetration depth of incident light is different for the various colors (wavelengths), having photodiodes with the same projection depth regardless of the intended spectral reception results in lower absorption coefficients for colors having longer wavelengths, e.g., red light. This variation in absorption causes unduly low levels at the red end of the spectrum and thus a color imbalance. Furthermore, with its longer wavelength and its deeper penetration, the absorption of red light may extend beyond the lower limits of the corresponding photodiode (active) region, and cause crosstalk, i.e., a transfer of charges between adjacent active regions.